Abstract

3502

ErbB (HER) receptors are often overexpressed in human malignancies and drugs targeted against ErbB1 and ErbB2 are currently used in cancer therapy. The tumorigenic role of ErbB4, however, remains controversial. Here, we analyzed the relevance of ErbB4 and its functionally distinct isoforms for expression, prognostic value and tumor promoting potential in breast cancer. Real-time RT-PCR analysis of 62 clinical breast cancer samples demonstrated selective overexpression of JM-a isoforms of ErbB4, compared to 20 normal humans tissues. ErbB4 JM-a isoform includes a protease recognition site, which enables it to be cleaved and translocated to nucleus where it may be involved in transcriptional regulation. The highest ErbB4 JM-a expression levels in breast cancer samples were 170-fold above the levels in normal mammary gland. This was in the same range as maximal overexpression of ErbB2 (215-fold) observed in patients with ErbB2 gene amplification. However, real-time PCR analysis revealed no cases with ErbB4 gene amplification (n=37). The overexpression of ErbB4 JM-a correlated with estrogen receptor-α (ER) positivity (p<0.001), and ErbB3 overexpression (p<0.001). To evaluate the prognostic value of ErbB4 JM-a expression, a tissue array consisting of 458 clinical breast cancer samples was analyzed by immunohistochemistry. Patients with high to moderate overall staining intensity for ErbB4 (46% of patients) had a tendency for prolonged survival (p=0.08). However, nuclear ErbB4 immunoreactivity (20% of patients), indicating nuclear translocation of the proteolytic carboxy-terminal fragment of ErbB4 JM-a, associated with significantly shorter survival when compared to cell surface ErbB4 immunoreactivity (p=0.04). To assess the functional significance of overexpression of the cleavable ErbB4 JM-a isoform for tumor formation, MCF-7 breast cancer cells were transfected with ErbB4 JM-a cDNA. ErbB4 JM-a overexpression promoted anchorage-independent growth in vitro and tumor formation in nude mice in vivo. The growth promoting effect was synergistic with the effect of exogenous estradiol. These findings demonstrate that human breast cancer tissues selectively overexpress ErbB4 JM-a, and suggest an oncogenic role for ErbB4 isoforms susceptible for cleavage and nuclear translocation.